FIG. 1 is a sectional view showing an example of a liquid discharge device 1 serving as a piezoelectric ink jet head used for an on-demand type ink jet printer or the like. FIG. 2 is a partially enlarged sectional view of a piezoelectric actuator 7 of the liquid discharge device 1 shown in FIG. 1. Referring to FIGS. 1 and 2, the liquid discharge device 1 in this example includes a substrate 5 having a plurality of liquid drop discharge sections 4 arranged therein in a planar direction, each of the liquid drop discharge sections 4 having a pressure chamber 2 to be filled with ink and a nozzle 3 communicating with the pressure chamber 2 for discharging the ink within the pressure chamber 2 as an ink drop, and a plate-shaped piezoelectric actuator 7 including a piezoelectric ceramic layer 6 having a dimension covering the plurality of pressure chambers 2 in the substrate 5 and laminated on the substrate 5.
The piezoelectric actuator 7 is partitioned into a plurality of piezoelectric deformation regions 8 respectively disposed so as to correspond to the pressure chambers 2 and individually deflected and deformed in the thickness direction by individual application of drive voltages, and a binding region 9 disposed so as to surround the piezoelectric deformation regions 8 and prevented from being deformed by being fixed to the substrate 5. Furthermore, the piezoelectric actuator 7 in the illustrated example has a so-called unimorph type configuration including discrete electrodes 10 respectively formed for the pressure chambers 2 on an upper surface of the piezoelectric ceramic layer 6 in both the drawings for defining the piezoelectric deformation regions 8, and a common electrode 11 and a vibrating plate 12 laminated in this order on a lower surface of the piezoelectric ceramic layer 6 and both having dimensions covering the plurality of pressure chambers 2. Each of the discrete electrodes 10 and the common electrode 11 are individually connected to a drive circuit 13, and the drive circuit 13 is connected to a control unit 14.
The piezoelectric ceramic layer 6 is formed of a piezoelectric material such as PZT, and is given piezoelectric deformation characteristics in a so-called transverse vibration mode by being previously polarized in the thickness direction of the layer. When a drive voltage in the same direction as the direction of the polarization is applied from the drive circuit 13 to an area between the discrete electrode 10 that define any one of the piezoelectric deformation regions 8 and the common electrode 11, an active region 15, which corresponds to the piezoelectric deformation region 8 and is sandwiched between both the electrodes 10 and 11, contracts in the planar direction of the layer, as indicated by transverse white arrows in FIG. 2. However, the lower surface of the piezoelectric ceramic layer 6 is fixed to the vibrating plate 12 through the common electrode 11. When the active region 15 contracts, therefore, the piezoelectric deformation region 8 in the piezoelectric actuator 7 is accordingly deflected and deformed so as to project toward the pressure chamber 2, as indicated by a downward white arrow in FIG. 2. When the piezoelectric deformation region 8 is vibrated by combining a state where the piezoelectric deformation region 8 is deflected and deformed and a state where the application of the drive voltage is stopped to release the deflection and deformation, the ink filled in the pressure chamber 2 is pressurized by the vibration and is discharged as an ink drop through the nozzle 3.
In the liquid discharge device, a so-called Pull-push driving method is generally employed widely, as disclosed in Patent Document 1. FIG. 3 is a graph showing a relationship between an example of a drive voltage waveform (indicated by a thick one-dot and dash line) generated by ON/OFF control of a drive voltage VP applied to the piezoelectric actuator 7 from the drive circuit 13 when the liquid discharge device 1 shown in FIG. 1 is driven by the normal Pull-push driving method, and a change in volume velocity of ink [indicated by a thick solid line, where (+) is on the side of the tip of the nozzle 3, that is, on the side of discharge of an ink drop, and (−) is on the side of the pressure chamber 2] within the nozzle 3 occurring when the drive voltage waveform is applied.
Referring to FIGS. 1 to 3, in a waiting time period during which no ink drop is discharged from the nozzle 3 on the left of t1 in FIG. 3, the drive voltage VP is maintained at ON satate, that is, at VH (VP=VH), to cause the active region 15 to continues to contract in the planar direction, to maintain a state where the piezoelectric deformation region 8 is deflected and deformed so as to project toward the pressure chamber 2, thereby to decrease the volume of the pressure chamber 2. During this period, the ink is in a stationary state, that is, the volume velocity of the ink in the nozzle 3 is maintained at zero, so that an ink meniscus formed by the surface tension of the ink remains stationary within the nozzle 3.
In order to discharge the ink drop from the nozzle 3 to form a dot on a paper surface, the drive voltage VP is turned off, that is, electrically discharged (VP=0V), at the time point of t1 immediately before that to release the contraction in the planar direction of the active region 15, to release the deflection and deformation of the piezoelectric deformation region 8. Thus, the volume of the pressure chamber 2 is increased by a predetermined amount. Therefore, the ink meniscus within the nozzle 3 is pulled toward the pressure chamber 2 by the amount of increase in the volume. The volume velocity of the ink within the nozzle 3 at this time gradually decreases after increasing once toward the (−) side, to come closer to zero in time, as shown in a portion between t1 and t2 in FIG. 3. This corresponds to a period that is substantially one-half an intrinsic vibration period T1 of intrinsic vibration of the volume velocity of the ink, indicated by the thick solid line.
Then, at the time point of t2 where the volume velocity of the ink in the nozzle 3 comes as close to zero as possible, the drive voltage VP is turned on, that is, electrically charged to VH (VP=VH) again to cause the active region 15 to contract in the planar direction, to deflect and deform the piezoelectric deformation region 8. As a result, the ink within the nozzle 3 is accelerated toward the tip of the nozzle 3 to project greatly outward from the nozzle 3 because the pressure of the ink pushed out of the pressure chamber 2 by deflecting and deforming the piezoelectric deformation region 8 to decrease the volume of the pressure chamber 2 is applied when the ink meniscus attempts to return to the tip of the nozzle 3 conversely from a state where it is pulled most greatly toward the pressure chamber 2 (a state where the volume velocity is zero at the time point of t2). At this time, the volume velocity of the ink within the nozzle 3 gradually decreases after increasing once toward the (+) side, to come closer to zero in time, as shown in a portion between t2 and t3 in FIG. 3. The ink that has projected outward from the nozzle 3 looks substantially columnar. Therefore, the ink in the projecting state is generally referred to as an ink column.
After a time point where the volume velocity of the ink in the nozzle 3 reaches zero (a time point of t3 in FIG. 3), the vibration velocity of the ink is directed to the pressure chamber 2, so that the ink column that has completely extended outward from the nozzle 3 is separated, to form an ink drop. The formed ink drop flies to a paper surface disposed so as to be opposed to the tip of the nozzle 3, to form a dot on the paper surface. The above-mentioned series of operations corresponds to application, to the piezoelectric deformation region 8, of the drive voltage VP having a drive voltage waveform including one pulse whose pulse width T2 is approximately one-half the intrinsic vibration period T1, as indicated by the thick one-dot and dash line in FIG. 3. When one dot is formed by two or more ink drops, the pulses described above, whose number corresponds to the number of ink drops, may be continuously generated.
Patent Document 1: Japanese Unexamined Patent Publication No. 02-192947 (Page 3 upper left column line 19 to page 3 upper right column line 6, page 3 upper right column line 14 to page 3 lower left column line 2, and FIG. 16(b)).